One problem in connection with cleaning of crankcase gases in this manner is to maintain a desired gas pressure in the crankcase. A solution to this problem is disclosed in EP1532353 B1 which describes the initially defined device in which the centrifugal rotor is arranged by its rotation to suck crankcase gas from the crankcase to the centrifugal separator, wherein the control equipment is arranged to change the rotational speed of the centrifugal rotor in response to a sensed change of said parameter in a way such that the gas pressure in the crankcase is maintained at a predetermined value, or at a predetermined pressure interval, during the operation of the combustion engine. Accordingly, the known device maintains a desired gas pressure within the crankcase by changing the rotational speed of the centrifugal rotor. However, reducing the rotational speed of the centrifugal rotor will also reduce the separating efficiency of the centrifugal rotor.